1. Field of the Invention
The present invention relates to a method of forming a silicon containing insulating film by the plasma CVD method.
2. Description of the Prior Art
Conventionally, the silicon containing insulating film formed by the plasma CVD method has, in general, the merit of a density that provides a low moisture permeability, and the merit that the amount of moisture in the insulating film is small.
Thus, such an insulating film has often been used as a covering film for an insulating film which has a considerably larger moisture permeability but possesses good step coverage.
A conventional method of forming the silicon containing insulating film by the plasma CVD method will be described with reference to FIGS. 1A to 1D which are cross sectional views showing an interlayer insulating film having a three layered structure, which is formed by sandwiching the silicon oxide film formed by the thermal CVD method between silicon containing insulating films formed by the plasma CVD method. In this case, a parallel plate type plasma CVD apparatus is used as the plasma CVD apparatus.
FIG. 1A is a cross sectional view showing a structure obtained after interconnection layers are formed and before the silicon containing insulating film is formed by the plasma CVD method.
In FIG. 1A, a numeral 1 denotes a silicon substrate; 2, a backing insulating film on the silicon substrate 1; and 3a and 3b, interconnection layers formed on the backing insulating film 2, spaced from each other by a predetermined distance. These layers constitute a deposition substrate 4.
As shown in FIG. 1B, first the deposition substrate 4 is placed in an evacuated chamber, then a reaction gas composed of a mixed gas of SiH.sub.4 +N.sub.2 O is introduced into the chamber for contact with the deposition substrate 4, and then the reaction gas is converted into a plasma by high frequency electric power (RF electric power). The N.sub.2 O gas is dissociated into activated nitrogen N* and activated oxygen O* which react with the SiH.sub.4 to form the silicon containing insulating film 5, i.e. a SiO.sub.x N.sub.y film covering the interconnection layers 3a, 3b. By adjusting x and y in the above SiO.sub.x N.sub.y film 5 through controlling reaction conditions, refractive index and etching speed, both closely related to denseness of the insulating film etc., can be controlled, and also film stress and the like can be adjusted. For instance, the refractive index can be controlled within a range 1.45 to 2.0 and can therefore be adjusted as required.
Then, as shown in FIG. 1C, a silicon oxide film (referred to as "NSG film" hereinafter) 6 is formed on the silicon containing insulating film 5 by the thermal CVD method using a mixed gas of TEOS+O.sub.3.
Next, as shown in FIG. 1D, the reaction gas, a mixture of SiH.sub.4 +N.sub.2 O, is converted to a plasma for reaction in the same manner as in FIG. 1B. Consequently, the silicon containing insulating film 7 made of SiO.sub.x N.sub.y is formed on the silicon oxide film 6.
Since the SiO.sub.x N.sub.y films 5 and 7, formed as described above, have inferior step coverage, they are not suitable for filling the recesses between the narrowly spaced interconnection layers 3a and 3b or for formation on a deposition substrate 4 having an uneven surface. Further, the SiH.sub.4 gas itself is well known as a highly dangerous gas.
Instead of a mixed gas of SiH.sub.4 +N.sub.2 O, a mixed gas of SiH.sub.4 +O.sub.2, may be used. However, the gaseous mixture of SiH.sub.4 +O.sub.2 is not suitable for use in the plasma CVD method since it is highly reactive in gas phase.
In order to avoid the above drawback, a TEOS reaction gas has recently been used, instead of a reaction gas containing SiH.sub.4, in the plasma CVD method. For example, a reaction gas of TEOS/O.sub.2 offers various advantages over a reaction gas of SiH.sub.4 +O.sub.2, in that the SiO.sub.2 film (silicon containing insulating film) can be formed under any conditions, with high safety, and excellent step coverage of the formed insulating film can be achieved.
However, since an organic compound is used as a reactant, there are many drawbacks in that the formed SiO.sub.2 film contains carbon and considerable moisture, and it has a high moisture permeability.
Although the SiO.sub.2 film can be formed under any conditions, it is hard to control the refractive index and the film stress of the insulating film. Thus the insulating film formed using TEOS as a reactant lacks flexibility in practical applications. For instance, the refractive index is always about 1.45.
Furthermore, in the case illustrated in FIG. 1D, the interlayer insulating film having a three layered structure, which is formed by sandwiching tile silicon oxide film formed by the thermal CVD method between silicon containing insulating films formed by the plasma CVD method, the silicon containing insulating film formed from TEOS/O.sub.2 is less conforming to the silicon oxide film, formed as the interlayer film by reaction of TEOS+O.sub.3, than the silicon containing insulating films 5, 7 formed by reaction of SiH.sub.4 +N.sub.2 O.
Namely, when the silicon oxide film formed by the thermal CVD method is formed on the silicon containing insulating film formed from the TEOS/O.sub.2 reaction system, the rate of forming the film is lower than that of forming the silicon oxide film directly on the silicon substrate and, therefore, unevenness in thickness occurs. Thus, anomalous growth of the silicon oxide film by the thermal CVD method is readily caused.
In addition, when a silicon oxide film is formed by the thermal CVD method on the silicon containing insulating film from a reaction gas of TEOS/O.sub.2, the step coverage by the silicon oxide film becomes poor. As a result, voids are often formed in the recesses between the interconnection layers etc., which are covered by such silicon containing insulating film.